Messages in this thread | ![/](/images/icornerl.gif) | | Date | Thu, 16 Mar 2023 23:21:07 +0000 | From | "Russell King (Oracle)" <> | Subject | Re: [PATCH v4 1/2] function_graph: Support recording and printing the return value of function |
| |
On Wed, Mar 15, 2023 at 06:39:10AM -0700, Donglin Peng wrote: > diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig > index e24a9820e12f..ad03fc868f34 100644 > --- a/arch/arm/Kconfig > +++ b/arch/arm/Kconfig > @@ -99,6 +99,7 @@ config ARM > select HAVE_FTRACE_MCOUNT_RECORD if !XIP_KERNEL > select HAVE_FUNCTION_ERROR_INJECTION > select HAVE_FUNCTION_GRAPH_TRACER > + select HAVE_FUNCTION_GRAPH_RETVAL if HAVE_FUNCTION_GRAPH_TRACER > select HAVE_FUNCTION_TRACER if !XIP_KERNEL > select HAVE_GCC_PLUGINS > select HAVE_HW_BREAKPOINT if PERF_EVENTS && (CPU_V6 || CPU_V6K || CPU_V7) > diff --git a/arch/arm/kernel/entry-ftrace.S b/arch/arm/kernel/entry-ftrace.S > index 3e7bcaca5e07..0151d2ce9958 100644 > --- a/arch/arm/kernel/entry-ftrace.S > +++ b/arch/arm/kernel/entry-ftrace.S > @@ -258,6 +258,10 @@ ENDPROC(ftrace_graph_regs_caller) > #ifdef CONFIG_FUNCTION_GRAPH_TRACER > ENTRY(return_to_handler) > stmdb sp!, {r0-r3} > +#ifdef CONFIG_FUNCTION_GRAPH_RETVAL > + /* Pass the function return value to ftrace_return_to_handler */ > + mov r1, r0
In a similar vein to Peter's comment, do we care about 64-bit return values here, because the above only covers 32-bit values.
If we do care about 64-bit values, then we get into EABI/OABI stickyness, because on EABI the 64-bit value would have to be passed in r2,r3, and OABI would need r1,r2.
it would be better to have the 64-bit argument as the first argument to ftrace_return_to_handler() which would eliminate that variability, but I don't know what effect that would have for other architectures.
Things get more icky if we want 128-bit values. For EABI, we've conveniently just stacked that. For OABI, that would need to be in r1-r3 and the final high bits on the stack.
With a 128-bit argument as the first, that would be r0-r3 with the existing stack pointer argument stored... on the stack.
So, really it depends what size of return value we want to report. Also, please bear in mind that where a function returns a 32-bit value, that will be in r0, and r1 will be whatever happened to be in it at function exit - there's no defined value for r1.
-- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!
| ![\](/images/icornerr.gif) |